1. Field of the Invention
The present invention relates to optical polarizers in general, and more particularly, to polarizers for high numerical aperture lithography.
2. Background of the Invention
A lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device generates a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising one or more dies) on a substrate (silicon wafer) that has been coated with a layer of radiation sensitive material (resist). In general, a single wafer or substrate will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time.
The term “patterning device” as here employed should be broadly interpreted as referring to device that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate. The term “light valve” can also be used in this context. Generally, the pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device.
An example of such a patterning device is a mask. The concept of a mask is well known in lithography, and it includes mask types such as binary, alternating phase shift, and attenuated phase shift, as well as various hybrid mask types. Placement of such a mask in the radiation beam causes selective transmission (in the case of a transmissive mask) or reflection (in the case of a reflective mask) of the radiation impinging on the mask, according to the pattern on the mask. In the case of a mask, the support will generally be a mask table, which ensures that the mask can be held at a desired position in the incoming radiation beam, and that it can be moved relative to the beam if so desired.
Another example of a patterning device is a programmable mirror array. One example of such an array is a matrix-addressable surface having a viscoelastic control layer and a reflective surface. The basic principle behind such an apparatus is that, for example, addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as undiffracted light. Using an appropriate filter, the undiffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind. In this manner, the beam becomes patterned according to the addressing pattern of the matrix addressable surface.
An alternative embodiment of a programmable mirror array employs a matrix arrangement of tiny mirrors, each of which can be individually tilted about an axis by applying a suitable localized electric field, or by employing piezoelectric actuators. Once again, the mirrors are matrix addressable, such that addressed mirrors will reflect an incoming radiation beam in a different direction to unaddressed mirrors. In this manner, the reflected beam is patterned according to the addressing pattern of the matrix-addressable mirrors. The required matrix addressing can be performed using suitable electronics. In both of the situations described hereabove, the patterning device can comprise one or more programmable mirror arrays. More information on mirror arrays as here referred to can be seen, for example, from U.S. Pat. Nos. 5,296,891 and 5,523,193, and PCT publications WO 98/38597 and WO 98/33096. In the case of a programmable mirror array, the support may be embodied as a frame or table, for example, which may be fixed or movable as required.
Another example of a patterning device is a programmable LCD array. An example of such a construction is given in U.S. Pat. No. 5,229,872. As above, the support in this case may be embodied as a frame or table, for example, which may be fixed or movable as required.
For purposes of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask and mask table. However, the general principles discussed in such instances should be seen in the broader context of the patterning device as hereabove set forth.
In current apparatus, employing patterning by a mask on a mask table, a distinction can be made between two different types of machine. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion at once. Such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatus, commonly referred to as a step and scan apparatus, each target portion is irradiated by progressively scanning the mask pattern under the radiation beam in a given reference direction (the “scanning” direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction. Since, in general, the projection system will have a magnification factor M (generally <1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be seen, for example, from U.S. Pat. No. 6,046,792.
In a known manufacturing process using a lithographic projection apparatus, a pattern (e.g. in a mask) is imaged onto a substrate that is at least partially covered by a layer of radiation sensitive material (resist). Prior to this imaging, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement and/or inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemical, mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer and the overlay (juxtaposition) of the various stacked layers is performed as accurate as possible. For this purpose, a small reference mark is provided at one or more positions on the wafer, thus defining the origin of a coordinate system on the wafer. Using optical and electronic devices in combination with the substrate holder positioning device (referred to hereinafter as “alignment system”), this mark can then be relocated each time a new layer has to be juxtaposed on an existing layer, and can be used as an alignment reference. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book “Microchip Fabrication: A Practical Guide to Semiconductor Processing”, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4.
For the sake of simplicity, the projection system may hereinafter be referred to as the “lens.” However, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include components operating according to any of these design types for directing, shaping or controlling the beam of radiation, and such components may also be referred to below, collectively or singularly, as a “lens.” Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask tables). In such “multiple stage” devices the additional tables may be used in parallel or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures. Dual stage lithographic apparatus are described, for example, in U.S. Pat. Nos. 5,969,441 and 6,262,796.
Development of new tools and methods in lithography have lead to improvements in resolution of the imaged features patterned on a device, e.g. an IC. Tools and techniques in optical lithography continue to improve possibly leading to a resolution of less than 50 nm. This may be accomplished using relatively high numerical aperture (NA) lenses (greater than 0.75 NA), wavelengths down to 157 nm, and a plethora of techniques such as phase shift masks, non-conventional illumination and advanced photoresist processes.
The success of manufacturing processes at these sub-wavelength resolutions will rely on the ability to print low modulation images or the ability to increase the image modulation to a level that will give acceptable lithographic yield.
Typically, the industry has used the Rayleigh criterion to evaluate the resolution and depth of focus capability of a process. The resolution and depth of focus (DOF) are given by the following equations:Resolution=k1(λ/NA),andDOF=k2(λ/NA2),where λ is the wavelength of the illumination source and k1 and k2 are constants for a specific lithographic process.
Therefore, for a specific wavelength, as resolution is increased through the use of higher-NA tools, the depth of focus can decrease. The loss in DOF with high NA is well known. However, the polarization targets for high NA partially coherent systems have not been examined. According to the following equation:
      I    ⁡          (              r        ,                  Z          0                    )        =                    ∑        i            ⁢                                    ∫            s                                                                                          ⁢                              ⅆ            ρ                    ⁢                                          ⁢                      J            ⁡                          (                              ρ                0                            )                                            |          FT      ⁢                          ⁢              {                              O            ⁡                          (                              ρ                -                                  ρ                  0                                            )                                ⁢                                    P              i                        ⁡                          (              ρ              )                                ⁢                                    F              i                        ⁡                          (                              ρ                ,                z                            )                                ⁢                      H            ⁡                          (                              ρ                ,                                  Z                  0                                            )                                          
where the image I, in a given film such as a photoresist, is a function of position r and specific for a given focus position Z0. This equation is valid for all NAs and the image is the summation over all polarization states i. The integral is over the source distribution defined by J. The Fourier term within brackets represents the electric field distribution at the exit pupil. The four terms inside the bracket are, respectively, the object spectrum O of the reticle pattern, a polarization function P, a film function F and a pupil function H.
According to this equation, high NA imaging is intrinsically linked with the polarization state and the thin film structure, where the electric field coupling and the power absorbed by a photoresist film can be drastically altered. The power absorbed due to incident plane waves on a photoresist film are proportional to the exposure necessary to develop the film.
Studies by Donis G. Flagello et al. published under the title “Optical Lithography into the Millennium: Sensitivity to Aberrations, Vibrations and Polarization,” in the 25th Annual International Symposium on Microlithography, SPIE, Feb. 27–Mar. 3, 2002, Santa Clara, Calif., USA, have shown that two orthogonal polarization (Transverse Electric polarization TE and Transverse Magnetic Polarization TM) diverge extensively at high NA, up to a 25% power change. An imaging system would contain a multitude of incident angles, reducing this effect. However, alternating phase shift masks (PSMs) require a small partial coherence which restricts the total number of angles and thus could produce similar exposure changes.
Results have been obtained through simulation which show that a critical dimension difference from a completely polarized state and the unpolarized state depends on the numerical aperture NA. Results have also shown that dense lines with an alternating phase shift mask (PSM) is the most critical feature and this has been explained by the fact that the pupil configuration essentially produces 2-beam interference at the wafer level and this case tends to maximize the effects of polarization. If, for example, a numerical aperture of 0.85 (relatively high) is selected and one wanted to limit the systematic critical dimension CD error to less than 3%, the residual polarization should be limited to 10%. The critical dimension CD is the smallest width of a line or the smallest space between two lines permitted in the fabrication of a device. The simulation results also indicate the level of pupil filling and partial coherence can lessen the effects of polarization. This has been shown by the small polarization influence on the features using conventional illumination.
Therefore, as more phase masks are used and imaging technology that demands small coherence levels is used, newer metrology technologies for the lens may be required. For example, high NA polarization effects may result in extremely tight specifications on illumination polarization for lithography tools.
The advent of a resolution-enhancement technique (RET) called “liquid immersion” promises extending 157 nm optical lithography to well below 70 nm and possibly below 50 nm without changes in illumination sources (lasers) or mask technology. According to a Massachusetts Institute of Technology (MIT) article by M. Switkes et al. entitled “Immersion Lithography at 157 nm” published in J. Vac. Sci. Technology B 19(6), November/December 2001, liquid immersion technology could potentially push out the need for next-generation lithography (NGL) technologies such as extreme ultraviolet (EUV) and electron projection lithography (EPL). The liquid immersion technology involves using chemicals and resists to boost resolution. Immersion lithography can enhance the resolution of projection optical systems with numerical apertures up to the refractive index of the immersion fluid. The numerical aperture NA is equal to the product of the index n of the medium and the sinus of the half angle θ of the cone of light converging to a point image at the wafer (NA=n sin θ). Thus, if NA is increased by increasing the index n, the resolution can be increased (see equation: Resolution=k1(λ/NA)). However, as stated above, higher NA may result in extremely tight specifications on illumination polarization for lithography tools. Therefore, polarization plays an increased role in immersion lithography.